1. Field of the Invention
The present invention relates to novel piezoelectric resonators and electronic components containing the same, and more particularly, to a novel piezoelectric resonator which maximizes the effective use of the mechanical resonance of a piezoelectric member, and electronic components containing such a novel piezoelectric resonator, such as an oscillator, a discriminator, and a filter.
2. Description of the Related Art
FIG. 34 is a perspective view of a conventional piezoelectric resonator. A piezoelectric resonator 1 includes a single piezoelectric substrate 2 having, for example, a rectangular plate shape as viewed from above. The single piezoelectric substrate 2 is polarized in the thickness direction. On two opposite major surfaces of the single piezoelectric substrate 2, electrodes 3 are provided. When a signal is input between the electrodes 3, an electrical field is applied to the single piezoelectric substrate 2 in the thickness direction and the single piezoelectric substrate 2 vibrates in the longitudinal direction. In FIG. 35, there is shown a piezoelectric resonator 1 in which electrodes 3 are provided on two opposite major surfaces of a single piezoelectric substrate 2 having a square plate shape as viewed from above. The single piezoelectric substrate 2 of the piezoelectric resonator 1 is polarized in the thickness direction. When a signal is input between the electrodes 3 in the piezoelectric resonator 1, an electrical field is applied to the single piezoelectric substrate 2 in the thickness direction and the single piezoelectric substrate 2 vibrates in a square-type vibration mode (in the plane direction).
These piezoelectric resonators shown in FIGS. 34 and 35 are unstiffened type resonators, in which the vibration direction differs from the direction of polarization and the electrical field direction. The electromechanical coupling coefficient of such an unstiffened piezoelectric resonator is lower than that of a stiffened piezoelectric resonator in which each of the vibration direction, the direction of polarization, and the direction in which an electrical field is applied are the same.
An unstiffened piezoelectric resonator has a relatively small frequency difference .DELTA.F between the resonant frequency and the anti-resonant frequency. This leads to a drawback in which a frequency-band width in use is narrow when an unstiffened frequency resonator is used as an oscillator or a filter. Therefore, the degree of freedom and flexibility in resonator characteristics design is limited in such a piezoelectric resonator and electronic components including such resonators.
The piezoelectric resonator shown in FIG. 34 uses the first-order resonance in the longitudinal mode. Because of its structure, the piezoelectric resonator of FIG. 34 also generates large spurious resonances in odd-number-order harmonic modes, such as the third-order and fifth-order modes, and in a width mode. To suppress these spurious resonances, some solutions have been considered, such as polishing, increasing mass, and changing the shape of the electrode. These solutions increase manufacturing cost.
In addition, since the single piezoelectric substrate has a rectangular plate shape, the single substrate cannot be made thinner without sacrificing required strength. Therefore, the distance between the electrodes cannot be reduced and a capacitance between terminals cannot be increased. This makes it extremely difficult to achieve impedance matching with an external circuit. To form a ladder filter by alternately connecting a plurality of piezoelectric resonators in series and in parallel, the capacitance ratio of the series resonator to the parallel resonator needs to be made large in order to increase attenuation. Because a piezoelectric resonator has the shape and structural restrictions described above, however, large attenuation cannot be obtained.
In the piezoelectric resonator shown in FIG. 35, large spurious resonances such as those in the thickness mode and in the triple-wave mode in the plane direction are generated. Since the piezoelectric resonator must have a large size as compared with a piezoelectric resonator using the longitudinal vibration in order to obtain the same resonant frequency, it is difficult to reduce the size of the piezoelectric resonator shown in FIG. 35. When a ladder filter is formed by a plurality of piezoelectric resonators, in order to increase the capacitance ratio between the series resonator and the parallel resonator, the resonators connected in series must have an increased thickness and electrodes are formed only on part of a piezoelectric substrate to make the capacitance small. In this case, since the electrodes are only partially formed, the difference .DELTA.F between the resonant frequency and the anti-resonant frequency as well as the capacitance is reduced. The resonators connected in parallel are accordingly required to have small .DELTA.F. As a result, the piezoelectricity of the piezoelectric substrate is not effectively used, and the transmission band width of the filter cannot be increased.
The inventors developed a piezoelectric resonator having small spurious resonance and a large difference .DELTA.F between the resonant frequency and the antiresonant frequency. In such a piezoelectric resonator, a plurality of piezoelectric layers and a plurality of electrodes are alternately laminated to a base member, and the plurality of piezoelectric layers are polarized in the longitudinal direction of the base member. This laminated piezoelectric resonator is a stiffened type resonator, and the piezoelectric layers are arranged such that the vibration direction, the direction of polarization, and the direction in which an electric field is applied are the same. Therefore, as compared with an unstiffened piezoelectric resonator, in which the vibration direction differs from the direction of polarization and electric field, the stiffened piezoelectric resonator has a larger electromechanical coupling coefficient and a larger frequency difference .DELTA.F between the resonant frequency and the antiresonant frequency. In addition, vibrations in modes such as the width and thickness modes, which are different from the basic vibration, are unlikely to occur in a stiffened piezoelectric resonator.
Since in a piezoelectric resonator having this lamination structure, each piezoelectric layer constituting the base member has the same length in the longitudinal direction of the base member and each electrode has the same area, the capacitance between each pair of adjacent electrodes is the same and the driving force piezoelectrically generated by each piezoelectric layer is also the same.
In longitudinal basic vibration, a stronger driving force is required at a portion located closer to the center of the base member in the longitudinal direction because of the large mass extending from this portion to an end of the base member in the longitudinal direction. Therefore, the piezoelectric resonator has an insufficiently large electromechanical coupling coefficient and thus .DELTA.F is not sufficiently large.
In the piezoelectric resonator, high-order-mode vibration is unlikely to occur. Since the capacitance between each pair of adjacent electrodes is constant because each piezoelectric layer has the same length in the longitudinal direction of the base member and each electrode has the same area, however, charges generated in each piezoelectric layer by odd-number-high-order-mode vibration, such as the third-order and fifth-order vibrations, are not sufficiently canceled, causing high-order-mode spurious vibrations.